U.S. patent application number 12/143188 was filed with the patent office on 2009-12-24 for fuel cell interconnect structures, and related devices and processes.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Matthew Joseph Alinger, Frederic Joseph Klug, Stephane Renou, James Anthony Ruud, Reza Sarrafi-Nour.
Application Number | 20090317705 12/143188 |
Document ID | / |
Family ID | 41050498 |
Filed Date | 2009-12-24 |
United States Patent
Application |
20090317705 |
Kind Code |
A1 |
Alinger; Matthew Joseph ; et
al. |
December 24, 2009 |
FUEL CELL INTERCONNECT STRUCTURES, AND RELATED DEVICES AND
PROCESSES
Abstract
A method for the formation of a diffusion barrier layer on a
surface of at least one fuel cell interconnect structure is
described. The interconnect structure is usually formed from
ferritic stainless steel, and includes chromium. The method
includes the step of coating an austenite phase-stabilizer on the
interconnect surface, and then heating the coated surface. The heat
treatment transforms the microstructure of the surface region of
the interconnect, from a substantially ferritic body-centered cubic
(BCC) phase to a substantially austenitic face-centered cubic (FCC)
phase. The diffusion rate of chromium through the FCC phase is
relatively low. Thus, the formation of a thick layer of chromium
oxide can be minimized, leading to better fuel cell performance.
Related fuel cells and fuel cell stacks are also disclosed.
Inventors: |
Alinger; Matthew Joseph;
(Albany, NY) ; Klug; Frederic Joseph;
(Schenectady, NY) ; Ruud; James Anthony; (Delmar,
NY) ; Renou; Stephane; (Clifton Park, NY) ;
Sarrafi-Nour; Reza; (Clifton Park, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
41050498 |
Appl. No.: |
12/143188 |
Filed: |
June 20, 2008 |
Current U.S.
Class: |
429/160 ;
427/115; 427/120; 429/221; 429/223; 429/224; 429/430; 429/433;
429/456; 429/535 |
Current CPC
Class: |
H01M 8/0228 20130101;
H01M 8/021 20130101; Y02E 60/50 20130101; H01M 2008/1293
20130101 |
Class at
Publication: |
429/160 ;
427/115; 427/120; 429/33; 429/224; 429/223; 429/221 |
International
Class: |
H01M 2/20 20060101
H01M002/20; H01M 2/00 20060101 H01M002/00; H01M 4/50 20060101
H01M004/50; H01M 4/52 20060101 H01M004/52; H01M 4/38 20060101
H01M004/38; H01M 8/10 20060101 H01M008/10 |
Claims
1. A method for the formation of a diffusion barrier layer on a
surface of at least one fuel cell interconnect structure formed of
a material comprising ferritic steel, comprising the following
steps: (a) applying a coating of an austenite-phase stabilizer to
the surface of the interconnect; and (b) heating the coated surface
to diffuse the austenite-phase stabilizer into the surface, so that
a surface region of the interconnect structure is transformed from
a substantially ferritic body-centered cubic (BCC) phase to a
substantially austenitic face-centered cubic (FCC) phase, wherein
the FCC phase exhibits the characteristic of reducing the diffusion
rate of a metal atom, as compared to the diffusion rate of the
metal atom through the BCC phase.
2. The method of claim 1, wherein the interconnect is attached to a
cathode of the fuel cell.
3. The method of claim 1, wherein the interconnect structure
material comprises chromium.
4. The method of claim 3, where the diffusion barrier layer has the
characteristic of reducing the diffusion rate of chromium, as
compared to the diffusion rate of chromium through a BCC ferritic
material.
5. The method of claim 1, wherein the austenitic stabilizer
comprises at least one metal selected from the group consisting of
nickel, cobalt, nitrogen, carbon, and manganese.
6. The method of claim 1, wherein the austenite-phase stabilizer
comprises manganese, cobalt, or a combination of manganese and
cobalt.
7. The method of claim 1, wherein the austenite-phase stabilizer is
cobalt.
8. The method of claim 1, wherein the coating of the
austenite-phase stabilizer is applied to the surface by a technique
selected from the group consisting of electroplating, electroless
plating, vacuum plasma spraying, low-pressure plasma spraying,
vacuum arc spraying, physical vapor deposition, electron beam
physical vapor deposition, sputter coating, and chemical vapor
deposition.
9. The method of claim 1, wherein the coated surface is heated in
step (b), under conditions sufficient to form a surface region
which has a depth of about 0.1% to about 10% of the thickness of
the interconnect structure.
10. The method of claim 9, wherein the surface region has a depth
of about 0.5 micron to about 10 microns.
11. The method of claim 1, wherein the surface region is heated to
a temperature which is at least about 40% of the melting point of
the ferritic steel material.
12. The method of claim 1, wherein at least a portion of the
heating of the coated surface is carried out during operation of
the fuel cell.
13. The method of claim 1, wherein the interconnect is attached to
an anode of the fuel cell.
14. A solid oxide fuel cell, comprising: (i) a cathode; (ii) an
anode; (iii) a ceramic electrolyte disposed between the anode and
the cathode, (iv) a cathode interconnect attached to an upper
surface of the cathode, having an interconnect surface which faces
and at least partially contacts a surface of the cathode; and (v)
an anode interconnect attached to a lower surface of the anode,
having an interconnect surface which faces and at least partially
contacts a surface of the anode; wherein at least one of the
cathode interconnect surface or the anode interconnect surface
comprises a surface region characterized by a substantially
austenitic face-centered cubic (FCC) phase.
15. The solid oxide fuel cell of claim 14, wherein the cathode
interconnect is formed of a material comprising ferritic stainless
steel, and includes an underlying bulk region characterized by a
substantially ferritic body-centered cubic (BCC) phase; and a
surface region characterized by the substantially austenitic
face-centered cubic (FCC) phase.
16. The solid oxide fuel cell of claim 15, wherein the average
depth of the surface region is in the range of about 0.5 micron to
about 10 microns.
17. The solid oxide fuel cell of claim 14, wherein the ceramic
electrolyte comprises a material selected from the group consisting
of zirconia, ceria, hafnia, bismuth oxide, lanthanum gallate,
thoria, and combinations thereof.
18. The solid oxide fuel cell of claim 14 wherein the anode
comprises a material selected from the group consisting of a noble
metal, a transition metal, a cermet, a ceramic, and combinations
thereof.
19. The solid oxide fuel cell of claim 14, wherein the cathode
comprises a material selected from the group consisting of
strontium doped LaMnO.sub.3, strontium doped PrMnO.sub.3, strontium
doped lanthanum ferrites, strontium doped lanthanum cobaltites,
strontium doped lanthanum cobaltite ferrites, strontium ferrite,
SrFeCo.sub.0.5O.sub.x, SrCo.sub.0.8Fe.sub.0.2O.sub.3-.delta.;
La.sub.0.8Sr.sub.0.2Co.sub.0.8Ni.sub.0.2O.sub.3-.delta.;
La.sub.0.7Sr.sub.0.3Fe.sub.0.8Ni.sub.0.2O.sub.3-.delta.; and
combinations thereof.
20. The solid oxide fuel cell of claim 15, wherein the cathode
interconnect material comprises: about 60 weight % to about 85
weight % iron; about 0 weight % to about 0.1 weight % carbon; about
15 weight % to about 30 weight % chromium; about 0 weight % to
about 1 weight % manganese; about 0 weight % to about 1 weight %
yttrium; and about 0 weight % to about 1 weight % lanthanum.
21. The solid oxide fuel cell of claim 15, wherein the
coefficient-of-thermal-expansion (CTE) of the cathode interconnect
is substantially identical to the CTE of the ceramic electrolyte
membrane.
22. A solid oxide fuel cell stack formed of a plurality of
interconnected fuel cells, wherein at least one of the fuel cells
comprises a cathode interconnect formed of a ferritic stainless
steel material and having a cathode interconnect surface facing the
surface of a cathode of the fuel cell; wherein the cathode
interconnect surface includes a surface region characterized by a
substantially austenitic face-centered cubic (FCC) phase.
Description
BACKGROUND OF THE INVENTION
[0001] The invention generally relates to fuel cells. More
specifically, the invention is directed to interconnect structures
and materials for solid oxide fuel cells.
[0002] Solid oxide fuel cells (SOFCs) are promising devices for
producing electrical energy from fuel with high efficiency and low
emissions. Like most fuel cells, the SOFC devices generate electric
current by the electrochemical combination of hydrogen and oxygen.
In a typical SOFC, an anodic layer and a cathodic layer are
separated by an electrolyte formed of a ceramic solid oxide.
Hydrogen, either pure or reformed from hydrocarbons, is flowed
along the outer surface of the anode, and diffuses into the anode.
Oxygen, typically from air, is flowed along the outer surface of
the cathode and diffuses into the cathode. Each O.sub.2 molecule is
split and reduced to two O.sup.-2 anions (catalytically) by the
cathode. The oxygen anions transport through the electrolyte and
combine at the anode/electrolyte interface with four hydrogen ions,
to form two molecules of water. The anode and the cathode are
connected externally through a load to complete the circuit,
whereby four electrons are transferred from the anode to the
cathode.
[0003] As shown in FIG. 1, an exemplary, planar SOFC 20 comprises a
cathode interconnect 22, and a pair of electrodes--a cathode 26 and
an anode 24. The cathode and the anode are separated by a ceramic
electrolyte 28. In general, this cell arrangement is well-known in
the art, although the configuration depicted in the figure may be
modified, e.g., with the anode layer above the electrolyte, and the
cathode layer below the electrolyte.
[0004] Commercial solid oxide fuel cell structures usually consist
of many of these cells stacked together--sometimes hundreds of
cells, which cumulatively provide enough voltage to make the device
commercially feasible. The cells are typically joined together by
interconnects, such as that noted above. The interconnects are
usually in the form of metallic or ceramic layers, and provide
electrical contact, current distribution, and structural integrity
between individual cells. In a typical cathode-electrolyte-anode
stack arrangement (viewed vertically for the sake of discussion),
one interconnect layer is attached to an upper surface of a cathode
layer, for connection to the anode layer of an adjacent cell or
"module". Another interconnect layer is attached to the lower
surface of the anode, for connection to the cathode layer of
another adjacent cell.
[0005] Metallic interconnects are often used in fuel cells, based
on considerations like cost and ease-of-fabrication. As in the case
of any type of interconnect (e.g., ceramic), the metallic alloy
composition must provide a desired level of hermeticity and
electrical conductivity under fuel cell operating conditions.
Moreover, the alloy material should be capable of withstanding the
effects of operation at high temperatures and temperature cycling
conditions.
[0006] In many cases, the cathode interconnect and the anode
interconnect are formed from a ferritic stainless steel material.
Ferritic stainless steels are well-known in the art, and are
usually based on iron, chromium, and various other selected
elements. Ferritic steels are useful for several reasons. As an
example, the materials are in the form of the body-centered cubic
(BCC) phase. Materials of this type have a coefficient of thermal
expansion (CTE) which can be closely matched to the CTE of the
electrolyte in the fuel cell. Matching of the expansion
characteristics for various layers in an SOFC is critical, in terms
of high-temperature electrical performance, and structural
integrity.
[0007] Ferritic stainless steels are certainly very useful as
interconnect materials. The chromium-containing alloys are easily
formed and shaped, and are also less costly than most of the
ceramic interconnect materials. However, the ferritic stainless
steels have some drawbacks as well. For example, the materials are
susceptible to thermally-induced oxide formation, i.e., the rapid
formation of a chromium oxide (chromia) layer on the surface of the
alloy component. In most cases, a thin, dense chromia layer on the
surface of the component may be beneficial for protecting the metal
surface, while also exhibiting relatively high electrical
conductivity. However, a fast-growing, thick chromia layer can
degrade the performance of the fuel cell, by increasing the overall
electrical resistance of the cell in a short period of time. As a
result, the useful life of the fuel cell can be decreased
considerably.
[0008] With these considerations in mind, new processes for the
fabrication of solid oxide fuel cells and fuel cell components
would be welcome in the art. The processes should result in fuel
cell structures which can provide optimum and stable
electrochemical characteristics and fuel efficiency over an
extended period of operation, e.g., as measured by the area
specific resistance (ASR) of the fuel cell. Moreover, the fuel cell
should exhibit good physical integrity and durability.
BRIEF DESCRIPTION OF THE INVENTION
[0009] The present invention meets these and other needs by
providing a method for the formation of a diffusion barrier layer
on a surface of at least one fuel cell interconnect structure
formed of a material comprising ferritic stainless steel. The
method comprises the following steps:
[0010] (a) applying a coating of an austenite phase-stabilizer to
the surface of the interconnect; and
[0011] (b) heating the coated surface to diffuse the austenite
phase-stabilizer into the surface, so that a surface region of the
interconnect structure is transformed from a substantially ferritic
body-centered cubic (BCC) phase to a substantially austenitic
face-centered cubic (FCC) phase.
[0012] In another embodiment of this invention, a solid oxide fuel
cell is disclosed. The fuel cell comprises:
[0013] (i) a cathode;
[0014] (ii) an anode;
[0015] (iii) a ceramic electrolyte disposed between the anode and
the cathode;
[0016] (iv) a cathode interconnect attached to an upper surface of
the cathode, having an interconnect surface which faces and at
least partially contacts a surface of the cathode; and
[0017] (v) an anode interconnect attached to a lower surface of the
anode, having an interconnect surface which faces and at least
partially contacts a surface of the anode;
[0018] wherein at least one of the cathode interconnect surface or
the anode interconnect surface comprises a surface region
characterized by a substantially austenitic face-centered cubic
(FCC) phase.
[0019] Yet another embodiment of this invention is directed to a
solid oxide fuel cell stack. The stack is formed of a plurality of
interconnected fuel cells. At least one of the fuel cells comprises
a cathode interconnect formed of a ferritic stainless steel
material and having a cathode interconnect surface facing the
surface of a cathode of the fuel cell. The cathode interconnect
surface includes a surface region characterized by a substantially
austenitic face-centered cubic (FCC) phase.
DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic representation of a solid oxide fuel
cell.
[0021] FIG. 2 is a schematic cross-section of a portion of a fuel
cell interconnect.
[0022] FIG. 3 is a depiction of the fuel cell interconnect of FIG.
2, to which an austenite phase-stabilizing material has been
applied.
[0023] FIG. 4 is a depiction of the fuel cell interconnect of FIG.
3, after a prescribed heat treatment.
DETAILED DESCRIPTION OF THE INVENTION
[0024] It should be noted that in the following description, like
reference characters designate like or corresponding parts
throughout the several views shown in the figures. It is also
understood that terms such as "top," "bottom," "outward," "inward,"
"first," "second," and the like are words of convenience, and are
not to be construed as limiting terms. Moreover, as used throughout
this disclosure, the terms "a" and "an" do not denote a limitation
of quantity, but rather denote the presence of at least one of the
referenced items. The suffix "(s)" as used herein is intended to
include both the singular and the plural of the term that it
modifies, thereby including one or more of that term (e.g., the
term "surface" may sometimes include one or more surfaces).
[0025] FIG. 1 was mentioned above, and describes a typical
structure for an SOFC, according to some embodiments of this
invention. Cathode interconnect 22 is disposed over and attached to
cathode 26. The inner surface 23 of the interconnect is formed from
a pattern of fuel flow channels 25, and includes both the walls of
the troughs, as well as the surface 27 of the "dividing walls",
which will directly contact cathode 26.
[0026] As alluded to above, the alloy composition for cathode
interconnect 22 is formed from a ferritic stainless steel alloy
(sometimes referred to herein as "ferritic steel"). Such alloys are
well-known in the art. Many of those which are used in
electrochemical cells comprise about 60 to about 85 weight % iron
and about 15 to about 30 weight % chromium. The alloys often
contain carbon (e.g., up to about 0.1 weight %) and/or manganese
(e.g., up to about 1 weight %). Various other metals may also be
included, such as yttrium and lanthanum, which are typically
present (in total) at a level of no more than about 1 weight %.
However, it should be noted that the present invention is
applicable to a wide variety of iron-chromium alloys which can be
characterized as "ferritic stainless steel".
[0027] As mentioned above, a coating of an austenite-phase
stabilizer is applied to the surface of the interconnect, i.e., to
inner surface 23 and surface 27. The austenite-phase stabilizer
comprises at least one metal selected from the group consisting of
nickel, cobalt, nitrogen, carbon, and manganese. In some specific
embodiments, the austenite-phase stabilizer comprises manganese,
cobalt, or a combination of manganese and cobalt. However, nickel
is preferred in many embodiments, while cobalt is the preferred
stabilizer in other embodiments. The austenite phase-stabilizer
usually must be deposited in metallic form. In some cases, though,
the stabilizer can be deposited in oxide form, if it is
subsequently reduced to metallic form, e.g., by a heat treatment in
a reducing atmosphere, such as a hydrogen furnace.
[0028] A variety of deposition techniques could be employed to
apply the austenite-phase stabilizer to the surface of the
interconnect. Non-limiting examples include electroplating,
electroless plating, vacuum plasma spraying, low pressure plasma
spraying, vacuum arc spraying; physical vapor deposition, electron
beam physical vapor deposition, sputter coating, and chemical vapor
deposition. In some embodiments, electroplating is the preferred
method of deposition; while in other embodiments, electroless
plating would be the technique-of-choice. Those skilled in the art
are familiar with these techniques, and can adapt each of them to a
particular deposition situation. The amount of austenite coating
material (i.e., the austenite-forming coating) which is to be
applied to the interconnect surface will be determined in part by
the desired depth of the austenitic surface layer or region, as
discussed below. In general, the thickness of the austenite-forming
coating is independent of the thickness of the substrate. Instead,
the coating thickness is optimized to reduce or impede diffusion
over the operating life of the fuel cell, as discussed herein.
[0029] After the austenite-phase stabilizer has been applied over
the surface of the interconnect (cathode interconnect, anode
interconnect, or both), a heat treatment is carried out to diffuse
the material into the surface. The particular heating conditions
will depend on a variety of factors, such as: the particular
austenite-phase stabilizer metal employed; the manner in which the
stabilizer metal was deposited; the specific composition of the
underlying ferritic steel material, its microstructural
characteristics; and the desired depth of the austenitic surface
region. Various production factors can also be important, e.g., the
amount of time required to form the desired surface region on an
interconnect structure, in a typical fabrication facility.
[0030] Usually, the interconnect surface region is heated to a
temperature which is at least about 40% of the melting point of the
ferritic stainless steel material. In some specific embodiments,
the surface region is heated to at least about 65% of the melting
point of the ferritic steel material. As a non-limiting
illustration in the case of a manganese or cobalt austenitic
element, the diffusion temperature will be in the range of about
600.degree. C. to about 1100.degree. C. In some specific
embodiments, the diffusion temperature will be in the range of
about 800.degree. C. to about 1000.degree. C. Those skilled in the
art understand that higher temperatures within these ranges may
result in shorter heat-treatment duration, while longer heat
treatments may compensate for lower diffusion temperatures. In some
preferred embodiments for a commercial setting, the heat treatment
time is usually in the range of about 1 hour to about 24 hours. (In
the case of in-situ heat treatments, as discussed below, the heat
treatment can actually be effected over the course of up to about
100 hours.) The heat treatment can be accomplished by a number of
techniques. It is usually carried out in a conventional furnace,
using an air or oxygen atmosphere. A reducing atmosphere (as
mentioned above) or an inert atmosphere could alternatively be
used.
[0031] In another embodiment of this invention, the heat treatment
for the interconnect surface region can be carried out "in-situ",
i.e., while the fuel cell is in operation. As one example, the
austenite phase-stabilizing material could be applied to the
interconnect, and then the interconnect could be incorporated into
the fuel cell structure. As the fuel cell reaches its initial
operating temperature (e.g., about 700.degree. C.-900.degree. C.),
phase transformation of the surface region of the interconnect will
usually begin to occur, as discussed below. Moreover, in some
embodiments, the heat treatment can be carried out by a combination
of initial, conventional heating and, then, in-situ heating. (As
used herein, the term "heating the coated surface" is meant to also
describe partial or total in-situ treatments).
[0032] As also mentioned previously, the heat treatment transforms
the surface region of the interconnect structure from a
substantially ferritic body-centered cubic (BCC) phase to a
substantially austenitic face-centered cubic (FCC) phase,
effectively forming a diffusion barrier layer. The average depth of
the surface region will depend on a number of factors, some of
which were mentioned above. They include: the particular austenitic
stabilizer metal employed; and the specific composition of the
underlying ferritic steel material.
[0033] In general, the surface region should be thick enough (i.e.,
its depth) to function as a barrier layer. The barrier layer
impedes the diffusion of chromium out of the interconnect surface,
and thereby reduces the oxidation rate of the base metal. However,
the surface region should be thin enough to ensure that the bulk
interconnect structure maintains a CTE value which is similar to or
substantially identical to other structural members of the fuel
cell, such as the ceramic electrolyte membrane.
[0034] In some embodiments, the surface region has a depth which is
in the range of about 0.1% to about 10% of the thickness of the
interconnect structure. A non-limiting illustration can be
provided, with reference to FIG. 1. For an interconnect having a
thickness ("x") in the range of about 120 microns to about 1500
microns, the austenitic surface region would typically have a depth
in the range of about 0.5 micron to about 10 microns. Those skilled
in the art will be able to select the most appropriate depth for a
surface region in a given situation, based on the teachings herein.
Moreover, various imaging techniques which are known in the art can
be used to determine the depth of the surface region which has
changed in terms of its microstructural phase. One example is
electron backscattered diffraction (EBSD) analysis.
[0035] The present disclosure also includes an inventive embodiment
which is directed to a solid oxide fuel cell (SOFC), as described
previously. With reference to FIG. 1, the exemplary planar fuel
cell comprises a cathode interconnect 22 and a pair of electrodes,
i.e., the cathode 26 and anode 24, separated by the electrolyte 28.
In general, this cell arrangement is well-known in the art.
However, the configuration depicted in the figure may be modified,
e.g., with the anode layer above the electrolyte, and the cathode
layer below the electrolyte. Those skilled in the art understand
that fuel cells may operate horizontally, vertically, or in any
orientation. In some cases, a bond layer may be situated between
interconnect 22 and cathode 26. (Moreover, for ease-of-viewing, the
thickness of the various layers in the figure is not necessarily
to-scale; and the layers are shown in an exploded view).
[0036] With continued reference to FIG. 1, interconnect portion 22
defines a plurality of airflow channels 25 in contact with the
cathode 26. As mentioned above, the austenite coating is applied to
the surface of the interconnect. Interconnect portion 33 (attached
to lower cathode 35) defines a plurality of fuel flow channels 34
in contact with the anode 24.
[0037] In some embodiments, it may also be desirable to apply an
austenite coating to the surface of the anode interconnect which
faces anode 24. Thus, the coating could be applied to the walls 45
(i.e., the troughs) of the fuel flow channels 34, as well as to the
surface 47 of the dividing walls of the structures. The
austenite-coated surface could then be heated as described
previously (for diffusion and phase transformation), either as a
separate step, or as part of another heat treatment for the fuel
cell. The use of the austenitic material on the anode interconnect
can also provide some of the key advantages noted herein for the
cathode interconnect. In the case of the anode interconnect, the
austenite-phase stabilizer is usually nickel.
[0038] In operating the fuel cell, a fuel flow 40 is supplied to
the fuel flow channels 34. An airflow 38, typically heated air, is
supplied to the airflow channels 25. The operation of a fuel cell
like that depicted in FIG. 1 is known in the art. As a non-limiting
example, U.S. patent application Ser. No. 11/565,236, filed on Nov.
30, 2006 for T. Striker et al, describes the general concepts
involved in the operation of a solid oxide fuel cell. U.S. patent
application Ser. No. 11/863,747, filed on Sep. 28 2007 for S. C.
Quek et al; as well as U.S. Pat. No. 6,949,307 (Cable et al) and
U.S. Pat. No. 6,296,962 (Minh), are also instructive. All of these
patents and patent applications are incorporated herein by
reference. In general, a fuel, such as natural gas, is fed to an
anode, where it undergoes an oxidation reaction. The fuel at the
anode reacts with oxygen ions (O.sup.2-) transported to the anode
across the electrolyte. The oxygen ions are reacted with hydrogen,
forming water, and releasing electrons to an external electric
circuit. As part of the fuel cell scheme, air is fed to the
cathode. As the cathode accepts electrons from the external
circuit, a reduction reaction occurs. The electrolyte conducts ions
between the anode and the cathode. The electron flow produces
direct current electricity, and the process produces heat and
certain exhaust gases and liquids, e.g., water or carbon
dioxide.
[0039] The compositions of the various structural layers of the
SOFC are known in the art. The ceramic electrolyte is typically
formed of a material capable of conducting ionic species (such as
oxygen ions or hydrogen ions), yet having relatively low electronic
conductivity. Examples of suitable ceramic materials include, but
are not limited to, various forms of zirconia, ceria, hafnia,
bismuth oxide, lanthanum gallate, thoria, and various combinations
of these ceramics. In certain embodiments, the ceramic electrolyte
comprises a material selected from the group consisting of
yttria-stabilized zirconia, rare-earth-oxide-stabilized zirconia,
scandia-stabilized zirconia, rare-earth doped ceria, alkaline-earth
doped ceria, rare-earth oxide stabilized bismuth oxide, and various
combinations of these compounds. In an exemplary embodiment, the
ceramic electrolyte comprises yttria-stabilized zirconia. Doped
zirconia is attractive because it exhibits substantially pure ionic
conductivity over a wide range of oxygen partial pressure levels.
In one embodiment, the ceramic electrolyte comprises a thermally
sprayed yttria-stabilized zirconia. One skilled in the art would
know how to choose an appropriate electrolyte, based on the
requirements discussed herein.
[0040] Similarly, the composition of the anode layer may depend on
the end use application. In one non-limiting embodiment, the anode
layer comprises a material selected from the group consisting of a
noble metal, a transition metal, a cermet, a ceramic, and
combinations thereof. Some examples of suitable anode materials
include, but are not limited to, nickel, a nickel alloy, cobalt,
nickel-yttria stabilized zirconia cermet, copper-yttria stabilized
zirconia cermet, nickel-ceria cermet, nickel-samaria doped ceria
cermet, nickel-gadolinium doped ceria cermet, and combinations
thereof. These anode materials may be doped with many different
cations. For instance, for zirconia, Y, Ca, Sc may be used as
dopants. In the case of ceria, Gd and Sm may be used as dopants. In
a particular embodiment, the anode layer comprises nickel. Nickel
provides the advantage of easy in-situ porosity formation, and is
very robust in the green state. Other advantages of nickel relate
to its relatively low cost and easy availability.
[0041] The cathode layer may also be formed from conventional
materials, such as a variety of electrically-conductive (and in
some cases ionically-conductive) compounds. Non-limiting examples
include strontium doped LaMnO.sub.3, strontium doped PrMnO.sub.3,
strontium doped lanthanum ferrites, strontium doped lanthanum
cobaltites, strontium doped lanthanum cobaltite ferrites, strontium
ferrite, SrFeCo.sub.0.5O.sub.x,
SrCo.sub.0.8Fe.sub.0.2O.sub.3-.delta.;
La.sub.0.8Sr.sub.0.2Co.sub.0.8Ni.sub.0.2O.sub.3-.delta.;
La.sub.0.7Sr.sub.0.3Fe.sub.0.6Ni.sub.0.2O.sub.3-.delta.; and
combinations thereof. Composites of these materials may also be
used. In certain embodiments, the ionic conductor comprises a
material selected from the group consisting of yttria-stabilized
zirconia, rare-earth-oxide-stabilized zirconia, scandia-stabilized
zirconia, rare-earth doped ceria, alkaline-earth doped ceria,
rare-earth oxide stabilized bismuth oxide, and various combinations
of these compounds.
[0042] Various other details regarding materials for fuel cell
components are known in the art. Moreover, methods for the
manufacture of the cells are also known. Those skilled in the art
are also familiar with techniques for the fabrication of fuel cell
stacks. In the exemplary embodiment shown in FIG. 1, the fuel cell
assembly 20 comprises a plurality of repeating units 30, usually
having a planar configuration. Multiple cells of this type may be
provided in a single structure. The structure may be referred to as
a "stack", an "assembly", or a collection of cells capable of
producing a single voltage output.
[0043] FIG. 2 is a schematic cross-section of a portion 50 of a
fuel cell interconnect, similar to that of cathode interconnect 22
(FIG. 1). The interconnect is formed of a ferritic steel material,
and includes surface 52, which typically faces a cathode layer (not
shown). In FIG. 3, an austenite material 54 (e.g., one comprising
cobalt or manganese) is applied over surface 52, as described
previously. The coated surface is then subjected to a heat
treatment (or allowed to be partially or fully heat-treated
in-situ), so as to diffuse the austenite-phase stabilizer into the
surface of the interconnect. As shown in FIG. 4, a surface region
56 is formed after the heat treatment. (The depth of the surface
region is enlarged somewhat for ease-of-viewing. Moreover, the
border of the region may actually be somewhat irregular, and may
also follow the pattern of a diffusion profile). As noted above,
the surface region is transformed into a substantially austenitic
FCC phase. The presence of the FCC phase can result in the
attendant advantages described herein, e.g., decreased chromium
diffusion from the fuel cell interconnect, and a consequential
increase in the useful service life of the cell. A residue of the
austenite-phase stabilizer may sometimes remain on the surface of
the interconnect after the heat-treatment. In some cases, the
residue may be beneficial, and is allowed to remain. In other
instances, it can be removed by a variety of cleaning techniques,
such as chemical etching or mechanical grinding.
[0044] The patentable scope of the invention is defined by the
claims. While this invention has been described in detail, with
reference to specific embodiments, it will be apparent to those of
ordinary skill in this area of technology that other modifications
of this invention (beyond those specifically described herein) may
be made, without departing from the spirit of the invention.
Accordingly, the modifications contemplated by those skilled in the
art should be considered to be within the scope of this invention.
Furthermore, all of the patents, patent publications, articles,
texts, and other references mentioned above are incorporated herein
by reference.
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